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arXiv 0807.3342astro-ph
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硅酸盐碳星BM Gem的不对称硅酸盐尘埃分布

Asymmetric silicate dust distribution toward the silicate carbon star BM Gem

Keiichi Ohnaka, Hideyuki Izumiura, Christoph Leinert, Thomas Driebe, Gerd Weigelt, Markus Wittkowski

备注：
7 pages, 3 figures, accepted for publication in A&A
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摘要：

硅酸盐碳星显示出10微米的硅酸盐辐射，尽管它们的光球富含碳。它们被认为有环双星或环伴星盘，作为过去质量损失所脱落的富氧物质的储存库。我们提出了在甚大望远镜干涉仪（VLTI）的硅酸盐碳星BM宝石的N波段光谱干涉观测。我们的目标是探测富氧尘埃的空间分布与高空间分辨率。使用UT 2-UT 3和UT 3-UT 4基线配置，在44- 62 m基线处用VLTI/VLBI观测BM Gem。BM Gem的N波段荧光强度在8 ~10 μ m范围内急剧下降，在10 μ m范围内逐渐增加，反映了亚微米级无定形硅酸盐颗粒的光学薄硅酸盐发射特征。在~44--46 m基线处获得的微分相位在~9和11微米之间的硅酸盐发射特征的中心部分显示出显著的非零值（~ -70度），揭示了硅酸盐发射区域的光心移位和不对称性质。观测到的N波段的vibration和微分相位可以用一个简单的几何模型来解释，在这个模型中，未分辨的恒星被一个具有方位亮度调制的环所包围。最佳拟合模型的特征是一个宽环（10微米处约70质量），其中明亮的区域与未解析的恒星偏移约20质量，位置角约为280度。这个模型可以被解释为一个系统的circum-companion盘，是一致的光谱特征的吸积盘周围的一个看不见的同伴最近发现的紫光谱BM宝石。

英文摘要：

Silicate carbon stars show the 10 micron silicate emission, despite their carbon-rich photospheres. They are considered to have circumbinary or circum-companion disks, which serve as a reservoir of oxygen-rich material shed by mass loss in the past. We present N-band spectro-interferometric observations of the silicate carbon star BM Gem using MIDI at the Very Large Telescope Interferometer (VLTI). Our aim is to probe the spatial distribution of oxygen-rich dust with high spatial resolution. BM Gem was observed with VLTI/MIDI at 44--62 m baselines using the UT2-UT3 and UT3-UT4 baseline configurations. The N-band visibilities observed for BM Gem show a steep decrease from 8 to ~10 micron and a gradual increase longward of ~10 micron, reflecting the optically thin silicate emission feature emanating from sub-micron-sized amorphous silicate grains. The differential phases obtained at baselines of ~44--46 m show significant non-zero values (~ -70 degrees) in the central part of the silicate emission feature between ~9 and 11 micron, revealing a photocenter shift and the asymmetric nature of the silicate emitting region. The observed N-band visibilities and differential phases can be fairly explained by a simple geometrical model in which the unresolved star is surrounded by a ring with azimuthal brightness modulation. The best-fit model is characterized by a broad ring (~70 mas across at 10 micron) with a bright region which is offset from the unresolved star by ~20 mas at a position angle of ~280 degrees. This model can be interpreted as a system with a circum-companion disk and is consistent with the spectroscopic signatures of an accretion disk around an unseen companion recently discovered in the violet spectrum of BM Gem.

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